Salt Power

Long ago, this substance was used as a tool of power. In future it will be used as a tool of power. We need to start talking about salt (all over again)!

If you haven’t yet read Material World, I suspect you might look at the six materials it features and wonder to yourself: “why salt?”

We all know instinctively that the modern world couldn’t function without iron or copper - we all know we need the structure of steel and the strength of steel tools, as well as the wiring that powers our lives.

We know we still need oil (though not perhaps how much of it we’ll still need even once we’re beyond 2050) and we know we need lithium for all those batteries we’ll be making in future. Everyone knows you need silicon - for semiconductors and for that matter to help form the concrete we’re all walking on.

But salt. Why salt? Well, if you’ve read the 3.5 chapters that constitute the salt section (that 0.5 is the bonus postscript) you’ll realise that salt isn’t merely a condiment, or for that matter an interesting history story. It’s also still at the heart of the modern industrial world.

You’ll know that we’re still treading ancient salt routes today - that much of the modern chemicals industry uses salt as its primary ingredient. Without salt we don’t have the chemicals we use to etch nanometre patterns onto silicon chips or to make paper or glass or for that matter the lithium compounds we put into electric car batteries.

You’ll have read, if you’ve gotten this far, about how modern methods to extract and refine lithium owe rather a lot to ancient methods to extract and refine salt. We are still tracing the footsteps of our Phoenician ancestors, even as we create 21st century products of all. This is the magic of the Material World - the thin, invisible, forgotten thread that connects us to our ancient forebears.

But one of the other dimensions of salt I managed only to mention in passing was its relevance for the energy transition. Because as the new age of green power dawns, it turns out that it won’t just be those whose countries sit over the lithium and cobalt, or those with the finest battery factories, who will be best placed. So too will those who have the salt.

And when you look through the crystalline prism of salt, it turns out there is one of those rarest of all things: a good news story for the UK…

Taller Than Blackpool Tower

There’s a bloody enormous underground hole directly beneath these pipes. It’s a spooky thought!

One of the more unsettling moments in researching the book came when I stood in a beautiful, green field in Cheshire, surrounded by cows, only to be told I was standing on top of the most almighty hole. For while I appeared to be standing on terra firma, I was actually atop an enormous salt mine.

The way we mine much of our salt these days is not with mechanical diggers (we do use those for rock salt, but the quantities are smaller) but by sending down water to do the job. We drill a hole down towards a slab of salt under the ground and pump down water and pressurised air. Then another pipe sends the resulting liquid - that water, infused with lots of salt (ergo brine) - back up to the surface.

That’s how we make most of our salt these days! At least in much of the developed world. And so a hundred or so metres underneath my feet was a hole taller than Blackpool tower and, well, much wider than Blackpool Tower. I was standing on top of a giant, hidden underground cavity bigger than a skyscraper.

A diagram of a salt cavern from this paper

In fact, much of Cheshire is pockmarked with these underground holes. As you’ll have read (or soon will - hint, hint) some of these underground “solution mines” have collapsed in years gone by, leaving large dents in the Cheshire countryside. These days, be assured, they are much smarter about how they do this and collapses are rare enough to be almost unheard of.

So what does this have to do with the bold new world of green power? Actually, it turns out, rather a lot. To see why let’s begin with a simple fact you’ll all be well aware of: renewable power is intermittent. The wind doesn’t always blow and the sun doesn’t always shine. So if you want a 100% renewable power grid then you need some form of backup for this intermittency.

Right now that backup effectively comes from gas power. The UK fires up its gas power stations to supplement its grid (and do balancing, which is a complex procedure ensuring it always has the right amount of power). This is clearly a fossil fuel solution, however.

Couldn’t you deploy batteries as a backup? Well, to some extent we’re already doing a bit of this but you’d need a hell of a lot of batteries to keep the entire grid going for more than a few hours. And given that we know there are longish periods of low wind every so often, battery storage is probably unlikely to be enough for the UK. So what to do?

One solution would be to have a lot of pump storage hydropower sites. I love pump storage. One of my favourite power stations (come on, we all have them!) is a pump storage site. The principle is simple: you have a big reservoir on a hill. When you have lots of power running through the grid you use that surplus power to pump water up to the top; when you run low on power you open the floodgates and use the falling water to turn a turbine and generate power.

But one problem with pump storage is the geological constraint. You need the right kinds of mountains with the right kind of topography in which to site these power stations. And even based on pretty optimistic assumptions, the total amount of power you could store in a suite of theoretical pump storage stations across Europe would probably not really enough. The second problem is that it’s nigh on impossible to get permission to make new reservoirs in many developed countries - the UK included. So we might need a different solution.

Where does the hydrogen go?

One of those solutions is hydrogen. Now, there are many problems with hydrogen as an energy source. For one thing, making it, especially in a carbon free way, is enormously energy inefficient. It involves running large, power hungry electrolysis cells which lose a lot of that energy along the way.

Another problem with hydrogen comes down to physics (or should this be chemistry? I am neither a chemist or physicist). Hydrogen molecules are very, very small and have a tendency to escape from containers very easily. One of the challenges with trying to replace natural gas with hydrogen is that while existing pipes are pretty good at holding methane in them, most of them would be unable to hold hydrogen - so you’d need to replace much of the grid.

So there are some big question marks about using hydrogen as a replacement for gas. However, most energy experts think it can play an important role as a backup to those intermittent grids. The principle is similar to those pump storage power stations. When the wind is blowing hard and there’s surplus power, you use that power to electrolyse water and turn it into hydrogen; when the wind is low, you then use that hydrogen as a power source, burning it in a power station, a little like we burn natural gas today. Voila: a completely decarbonised power grid!

But hydrogen is hard to store. You can use chemistry to turn it into ammonia - a solid if combustible substance, but that involves further wasted efficiency. Or you can try to find somewhere to put it. But where? These days we store a lot of our natural gas in old, disused oil and gas reservoirs. But you can’t do that with hydrogen because the gas, with all those little molecules, would seep into the porous rock surroundings.

All of which brings us back to salt. Salt is the best natural substance we know of which can successfully store hydrogen at high pressure. It’s not porous, it’s quite plastic (which means it tends to bulge rather than shattering) and we’ve also got lots of experience in making big holes under the ground from salt. Indeed, a good proportion of Europe’s natural gas - that part that’s not stored in old oil/gas reservoirs - is already stored in salt caverns like the one under my feet in Cheshire.

So in the future if we’re going to have lots of hydrogen as an energy storage solution (which seems quite likely) we will need to find lots of salt to store it in. And so, all of a sudden, we need to start thinking about where the salt is. And it turns out, while salt deposits are hardly uncommon, they are not exactly everywhere.

Consider this map of salt deposits around Europe. There are quite a lot in the North Sea and plenty in the UK and parts of Northern Europe, but not much in the north of France; none in Italy or in Austria or in much of Eastern Europe (though Poland has plenty). The salt is far from everywhere.

And actually not all of these salt deposits are equal. If you want to start making big holes in salt caverns in which to store hydrogen (and they should ideally be very big - we’re talking about underground holes as tall as the Shard - Britain’s tallest skyscraper) you need big, uniform deposits rather than ones with lots of rock and stone embedded in them. It turns out a lot of France’s salt deposits would be ruled out for this reason. Ideally the salt would be close to the shoreline so you can a) link them up to offshore wind power and b) so you can drain away some of the saltwater created in their production (though I wonder whether we couldn’t find something else useful to do with all that salt than increasing the salinity of the oceans?).

Anyway, tot it all up and work out where the most appropriate salt is around Europe - as geologists have done - and here’s what you find. Lots in Germany, lots in Denmark, lots in the Netherlands and Norway (though much of that is offshore which would be a bit more impractical and expensive to work with), lots in Poland (though most is a long way from the sea) and loads of salt in the UK.

Indeed, the UK has so much potential for salt cavern storage of hydrogen that it could store more than enough hydrogen for its own needs. Enough, potentially, to provide much of Europe’s energy. Moreover much of that salt is to be found on the very north east coast nearest to so many of those big wind farms. It’s just where you’d want it to be.

This isn’t much discussed these days, partly I think because people don’t talk enough about salt and so don’t realise that not every country sits on top of good, accessible salt formations. But it’s an important factor when it comes to the energy transition.

There are many challenges facing the UK but there are two important things it has in its favour: the North Sea, which is both one of the shallower seas in the world and also one of the windiest - making it perfect for offshore wind. And it has loads and loads of salt it could use to store green hydrogen made from North Sea power. There is no inherent reason why it couldn’t be a European energy superpower. Oh and by the way, America has loads of salt; indeed, the strategic petroleum reserve is largely stored in old salt domes underground.

It’s another reason why it’s worth spending a bit of time thinking about salt. This marvellous mineral isn’t just a thing of history (though the historical stories surrounding it are brilliant). It’s not just an important part of our diet and our pharmaceutical industry. It’s also the key to unlocking the green future. Spain may have more potential for solar than the UK, but it has less salt; Norway may have access to the North Sea as well, but its salt is all offshore rather than being readily accessible under rural areas on the land.

For centuries, salt was a store of wealth; it was used as a currency. For centuries it was used as a tool of power, controlled by despots who taxed and monopolised its trade In the future, salt will be central to storing energy. It never stopped being relevant, but it’s about to become even more important.